In order to provide complete suppression of voltage transients in electrical circuits, and particularly circuits that include hot, neutral, and ground conductors, it is desirable to provide surge suppression utilizing such components as metal oxide varistors, bridging each of three possible pairs of conductors in which these voltage transients can occur. More specifically, it is desirable to provide a surge protection device connected between the hot and ground lines, between the hot and neutral lines and between the neutral and ground lines.
Some forms of TVSS devices, such as varistors, are subject to failure during operation, either as a result of high energy transient voltages being applied to the devices, the repetitive application of low energy transient voltages being applied to the devices, or even the long term application of steady state voltages above the suppression threshold. Such varistors are subject to failure as a result of thermal or chemical breakdown of the components of the varistor. Any of these may lead to immediate varistor failure.
As the varistor fails or begins to fail, it's surface can begin to break down, become conductive and cause short-circuiting with other conductive parts of the TVSS product. As a result, the electronic components and other metal components must be shielded from each other and/or separated by distances large enough to prevent short circuiting and premature failure of the device. Such separation distances may limit the size and number of components that can be used in a receptacle with conventional dimensions.
The failure mode associated with metal oxide varistors, produces a short circuit or low impedance failure. That is, the impedance of the device is reduced significantly below its normal operating impedance, as a result of failure. It is known to protect electrical circuits against such failure, by providing fuses in series with the varistors so that if the varistors fail, the fuses open and the varistor does not itself cause a short circuit or low impedance condition on the power circuit, which could create a risk of overloading the circuit, leading to overheating, the tripping of remote circuit breakers, or the like.
When a metal oxide varistor fails in a way that causes a series connected fuse to open, the protection provided by the varistor is lost. Moreover, the failure may be undetectable by observation of the devices connected to the circuit. Therefore, it is possible for the transient voltage protection to be lost without any obvious signs thereof, and thereafter for voltage transients to be passed unsuppressed to the equipment connected to the previously protected circuit, possibly causing damage.
U.S. Pat. No. 4,872,081 discloses an electrical receptacle having integral surge suppressors operative for protecting electrical apparatus connected to the outlets from transient line voltage surges. The receptacle has front and rear casings forming an enclosed housing for a printed circuit board and carrying contacts into which the blades of a plug connected to the receptacle are inserted. The housing includes portions cooperatively placed with respect to the circuit board to provide underlying support for the contacts, as well as to assist in positioning the circuit board relative to the housing and to maintain UL spacing of the components. The circuit components include three varistor devices for suppressing high transient voltages and the housing includes internal walls providing recesses to receive the respective varistor devices. Such a receptacle lacks a physical barrier between the electronic components and the line contacts.
It is an object of this invention to provide a transient voltage surge suppressor (TVSS) that overcomes the problems mentioned before.
It is a more particular object to provide a TVSS that includes a separator which provides a physical barrier between adjacent electronic components and between electronic components and the receptacle contacts of the device.